1. Field of the Invention
The present invention relates to a lighting apparatus of edge light type used for backlighting of a light receiving type liquid crystal display device.
2. Description of the Prior Art
Conventionally, many examples of lighting apparatus for liquid crystal display devices have been disclosed in such Japanese Laid-open patent publications as Nos. 987/1989 and 171009/1991. In general, as shown in FIG. 1, in a conventional lighting apparatus of edge light type used for a liquid crystal display device, lamps such as cold cathode tubes or hot cathode tubes are used as light sources 1, the light sources 1 are disposed at opposite ends of an optical conductor 2, a diffusion sheet 3 for uniforming the brightness over the entire lighting surface, which is formed of a milk-white, synthetic resin plate using such as acryl, polycarbonate or polyester plate, is provided on the top surface of the optical conductor 2, and a mirror reflector plate or a light scattering acryl plate is used as a reflector sheet 4. The reflector sheet 4 is adapted to improve the utilization efficiency of light by reflecting light, emitted from the light sources and heading to the back side, toward the front side.
Especially, in the field of color LCD panels, it is indispensable to employ stacked polarizer plates which utilize optical anisotropy of liquid crystal and the polarizer plates will cut illumination light by about 50%; and therefore, a backlighting system of higher brightness is needed and it is of importance to improve the utilization efficiency of light.
Denoted in FIG. 1 by 7 is light source reflectors for reflecting, light emitted from the light sources and heading to the outside, toward the optical conductor 2 and by X is a liquid crystal display panel.
In the conventional liquid crystal lighting apparatus of edge light type wherein light is introduced from light incident end surfaces 5 at the opposite ends of the optical conductor 2, the light propagates through the optical conductor 2 while repeating total reflection. In case where an angle between light incident on a light outgoing surface 6 standing for the top surface of the optical conductor 2 and a normal vector of the light outgoing surface 6 is .theta..sub.o and the refractive index of the optical conductor 2 is n.sub.o, the light can go out of the light outgoing surface 6 only when EQU .theta..sub.o &lt;sin.sup.-1 (1/n.sub.o)
is valid. Accordingly, with EQU .theta..sub.o .gtoreq.sin.sup.-1 (1/n.sub.o)
held, the light repeats total reflection in the optical conductor 2 to travel therethrough while undergoing a decrease in flux of light and a material absorption subject to an absorption coefficient .alpha. of an optical conductor material during propagation in the optical conductor 2. More particularly, where an optical path length through which light travels before reaching the light outgoing surface is P and a lamp light flux value at the light incident end surface of the optical conductor is F.sub.o, the light undergoes an decrease in flux of light pursuant to EQU F=F.sub.o e.sup.-.alpha.P (lumen)
and as a result, the utilization efficiency of light is decreased, causing difficulties in realizing a lighting apparatus (backlighting) of high brightness, especially in the field of color LCD units.